next time it happens, go into 'Network Connections' and disable then re-enable your NIC, the site should be browsable now.
Similar thing happens to me all the time, I can't quite understand why it happens or what is causing it, but all of my systems are the same (they can't browse...) - which means it's not just my own machines NIC.
I was thinking DNS problem, and have since disabled XP's DNS cache and am running through my ADDS DNS server instead - which forwards non-internal requests to an external providers DNS servers...seems to be working so far
